A hands-free facility for telephones includes a circuit for controlling the dynamic range of the voice signals ("Elektrisches Nachrichtenwesen", Vol. 53 (1978), No. 4, pp. 288 to 293). It offers to the user a number of advantages: The conversation can be conducted without a handset, making it possible to use both hands during the call, possibly for taking notes or flicking through files, and so on. Also, the user has greater freedom of movement because he can now talk further from the telephone set. Since the voice of the distant party, the B-subscriber, is reproduced via the loudspeaker, other people can listen in to the conversation and can actively participate by speaking into the built-in sensitive microphone. These advantages will gain even greater significance in connection with the new communication technologies, such as video telephony, one of the services to be offered in the planned BISDN.
On the other hand, however, hands-free operation presents difficulties from the point of view of acoustics and transmission. Since all ambient noise is received and treated by the microphone in exactly the same way as the voice, the signal-to-noise ratio is lower than that with a handset which receives a well-defined direct microphone input. The same applies at the receive side, where the human ear hears both the useful signal and the interfering signal.
A signal received by the A-subscriber from the B-subscriber is reproduced as an acoustic signal by the loudspeaker of the hands-free facility. This acoustic signal propagates along the air path between the loudspeaker and the microphone and is received with a certain attenuation by the microphone, which sends it back to the B-subscriber. A closed loop is thus formed via the send path, the receive path, and the air path. If the gain of this loop is V=1/k, where k is the coefficient of coupling between the loudspeaker and the microphone, a constant ringing tone is produced. This is extremely irritating, and measures must be taken to avoid it.
The prior art hands-free facility therefore includes a voice control system by which the gain in both transmission paths is controlled as a function of the voice level. An increase in gain in the send path causes a corresponding decrease in gain in the receive path, and vice versa. An optimum value for the voice-controlled swing can be estimated as a function of the particular operation conditions.